Novak Djokovic lost one of his two opening matches at the Adria Tour, as elite men's tennis returned for the first time since being suspended due to the coronavirus pandemic.
The world number one, 33, cruised past fellow Serb Viktor Troicki 4-1 4-1 in Belgrade in just 34 minutes.
He then lost 4-2 2-4 1-4 to world number 32 Filip Krajinovic.
Djokovic set up the tournament to help players get back to match fitness after several weeks of no competition.
The cross-Balkan event has a reduced match format and attracted 4,000 fans.
But the tournament's third leg, scheduled to take place in Montenegro, has been cancelled because Serbian citizens are banned from entering the country amid the coronavirus pandemic.
Djokovic said "several alternative options and locations" are being looked at but added that they are "running out of time".
At one point during Djokovic's opening victory in Serbia's capital, Troicki enlisted the help of a ball boy, allowing him to return two of the 17-time Grand Slam singles champion's serves.
The evening session began with a shock as Serbian world number 145 Nikola Milojevic, who was a late replacement for the injured Damir Dzumhur, beat Bulgarian world number 19 Grigor Dimitrov 4-1 4-3.
Milojevic's compatriot Dusan Lajovic almost caused a similar upset against world number three Dominic Thiem, but the Austrian came from match point down to win 1-4 4-1 4-3 (8-6).
Germany's world number seven Alexander Zverev beat Serbia's Krajinovic 0-4 4-3 4-3 and then swept past Troicki 4-1 4-1.
The last round of group games, including Djokovic v Zverev, will be played on Sunday before a final in the evening.
The regular ATP Tour is suspended until at least 31 July.
How does the Adria Tour tournament work?
- Matches are best of three sets, with a player needing to win four games instead of six in each set, and a tie-break coming at 3-3.
- There is no 'advantage' played following deuce, instead the game is decided on a 'sudden-death' point.
- The next round takes place in Zadar, Croatia from June 20 and the event finishes in Bosnia-Herzegovina in July.
- Each event takes place across one weekend. Players face each other in a round robin format in two groups of four.
- The final will take place between the two group winners on the Sunday evening.
Meanwhile, rain has washed out the opening day of the Ultimate Tennis Showdown in Nice, France, which is set to feature several top 10 players, including Stefanos Tsitsipas, Matteo Berrettini and David Goffin.
Matches scheduled for Saturday will be played on Monday, while matches set for Sunday remain unchanged. The tournament was created by Serena Williams' coach Patrick Mouratoglou.